1. Field of the Invention
The present invention pertains to a carriage for positioning inspection apparatus within an electrical generator.
2. Background Information
The stator cores of turbo-generators and other electrical machines are built up of electrical grade steel laminations, each of which is coated with a thin layer of electrical insulation. This insulation prevents the 60 HZ alternating magnetic flux in the stator core from inducing eddy currents between laminations. In most turbo-generators, the laminations are electrically connected together at their outside diameter where they are supported by the stator frame. However, if the insulation is defective near the stator bore, a conducting path is formed through which currents are induced by the alternating flux. The heating from the induced currents is highest at the defective insulation contact points between laminations where current density is increased. These damaged regions may become hot in service and such regions are, typically, identified as "hot spots". Hot spots can also occur in generators in which the laminations are electrically insulated from the stator frame. In this case the electrical circuit is usually completed through defective interlaminar insulation near the core outside the diameter.
The interlaminar insulation may be damaged during assembly or maintenance of a stator, particularly during the removal and replacement of its rotor. Hot spots may also be caused by foreign objects, usually metallic, or by general deterioration of the interlaminar insulation. The hot spots can, if undetected, increase in magnitude to the point of iron melting and/or damaging the adjacent insulation surrounding the copper conductors.
Stator cores can be tested for damage using a "thermovision" test. In this test the core is excited by a massive winding to full rated flux. By this method any hot spots on the teeth are readily detected with an infra-red camera scanning the bore. However, this test is unlikely to detect deep-seated faults unless more sophisticated temperature measurements are made.
One type of apparatus which is suitable for detecting such stator core hot spots is an Electro-magnetic Core Imperfection Detector ("EL-CID"). With EL-CID, the stator core is excited to only about 3 percent of the full rated flux in the core, which is sufficient to induce eddy currents to flow in the damage insulation areas. Because the current is very small, however, the heating is insignificant. The EL-CID test relies on the electromagnetic detection of the axial fault currents flowing through the damaged region.
A special pick-up coil in the sensing head, known as a chattock potentiometer, is used to measure the magneto motive force between the teeth to detect any fault currents. The output of the coil is amplified and phase-sensitive detected to yield a D.C. voltage proportional to the component of fault current in phase quadrature with the excitation current. The chattock coil signal is referenced in the phase sensitive signal processor to a constant signal derived from a reference coil which is maintained at one position in the stator. The purpose of the reference signal is to cancel out the chattock coil signal produced by the excitation field and thereby increase discrimination with respect to the fault currents.
Once a hot spot has been found using EL-CID, very standard industry procedures exist for restoring the interlaminar resistance of the punchings. Included are etching techniques and iron spreading for the addition of mica insulation with localized iron replacement as the solution for extreme insulation damaged areas.
To effectively use such EL-CID apparatus without disassemble of a generator, an effective system is necessary for positioning the EL-CID apparatus at appropriate test locations.
Additionally, other generator problems develop which can be detected by visual inspection of the stator of the generator. Due to the inaccessibility of the generator stator, without rotor removal, such visual inspection is best made when a camera is positioned in the vicinity of the generator stator which is to be inspected. Again, to be effective, it is necessary that some system properly position the camera within the generator.
Existing tests to perform the inspections described above are conducted manually and require removal of the generator rotor from within the stator. Rotor removal is a long and involved process. Removing the rotor, manually performing inspections and replacing the rotor can often consume 10 to 14 days. Removing a generator from service for such a period imposes serious problems on those relying on operation of the generator. Further, removing the rotor from the stator can itself cause damage to the stator. Also, replacing the rotor can damage the stator and a stator that has passed inspection may, therefore, become damaged after inspection, during rotor replacement.
Accordingly, there exists a need for an inspection system for an electric generator that does not require removal of the rotor from within the stator to perform the inspection and which does not require that the generator employ any particular type of baffle wedge groove to be functional.